Essentially, there are two general types of aerobic waste water treatment processes.
The first is commonly known as the activated sludge or suspended growth system. In this process, dissolved oxygen is fed into a storage tank of sludge to be treated, biomass forming as free-floating colonies or aggregates known as "flocs". A clarification stage is usually added to this process to separate these flocs or sludge from the liquid component.
If necessary, some of the sludge is periodically removed to prevent excessive solids build-up. Recycling is also practiced, principally to selectively increase the residence time of the biomass over the hydraulic residence time. In this manner real time control of activated sludge systems is limited to flow control of the recycle sludge to maintain solids at constant levels, and control of dissolved oxygen injected into the system to reduce operating costs.
The major problem with these type of systems are that they are generally designed for maximum capacity and further that the effluent quality can not readily be controlled.
The other kind of treatment process is known as the supported growth system where the biomass grows on biologically inactive support material. When the layer of biomass gets past a certain thickness, some of the sludge is sheared off or sloughed and this debris has to be removed from the final effluent by a clarification stage. Recycling is generally also practiced to increase the liquid/biomass contact and to dilute the incoming fluid with a partially treated liquid. Real time control of supported growth systems has also not proved practical in the past.
In summary, the key differences between activated sludge processes and other biological waste water treatment processes is that, operated in accordance with the present art, only the activated sludge process can be operated to give a reasonable assurance of meeting set effluent standards in the face of considerably fluctuating feed loadings. Other systems, such as supported growth units, and lagoons, have been capable to date of only removing a constant proportion of the pollution feed rather than meeting with a predetermined effluent quality.
This means that all existing systems have to be designed either with gross excess capacity, or as multiple units in series, or to a combination of both of these principles. Although the activated sludge process is known to carry appreciable cost penalties both directly in power and construction cost, and indirectly in quantity and quality of process monitoring and control required, its ability to absorb surges in load has, for the last few years, made this the standard process chosen for waste water treatment.
However, the selected activated sludge process has an additional pronounced disadvantage in that intermediate levels of treatment are only possible under conditions that also make large quantities of sludge. Accordingly, such systems are far from ideal where discharges are to be controlled at intermediate standards, such as for discharge to a sewer, or where some level of pre-treatment is desired to reduce overloading, or to introduce biological nutrient removal at an existing plant.
There is therefore a need for a process that reliably and simply delivers an intermediate level of treatment without generating very large quantities of sludge.